1. Field of the Invention
The present invention relates to systems for starting motors. More particular, the present invention relates to a soft start system for reducing the power requirements of the motor during the starting of the motor. Additionally, the present invention relates to soft start systems which utilize variable frequency drives.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
Variable frequency drives (often abbreviated “VFD”) are systems for controlling the rotational speed of an alternating current (AC) electric motor by controlling the frequency of the electrical power supply to the motor. A variable frequency drive is a specific type of adjustable speed drive. Variable frequency drives are also known as adjustable frequency drives, variable speed drives, AC drives or inverter drives. A variable frequency drive system generally includes an AC motor, a controller and an operator interface. The motor used in a VFD system is usually a three-phase induction motor or a synchronous motor. The variable frequency drive controller is a solid state electronic power conversion device. The usual design first converts AC input power to DC intermediate power using a rectifier bridge. The DC intermediate power is then converted to quasi-sinusoidal AC power by using an inverter switching circuit. The rectifier is usually a three-phase diode bridge, but controlled rectifier circuits are also used. Currently, insulated gate bipolar transistors (IGBTs) are used on most VFD inverter circuits.
AC motor characteristics require the applied voltage to be proportionally adjusted whenever the frequency is changed. For example, if a motor is designed to operate at 460 volts at 60 Hz the applied voltages must be reduced to 230 volts when the frequency is reduced to 30 Hz. Thus, the ratio of volts per Hz must be regulated to a constant value. One method used for adjusting the motor voltage is called pulse width modulation (PWM). With PWM voltage control, the inverter switches are used to divide the quasi-sinusoidal output waveform into a series of narrow voltage pulses. The inverter switches to modulate the width of the pulses. An embedded microprocessor governs the overall operation of the VFD controller.
The operator interface provides a means for an operator to start and stop the motor and adjust the operating speed. Additional operator control functions include reversing and switching between manual speed adjustment and automatic control from an external process control signal. The operator interface often includes an alphanumeric display and/or indication lights and meters to provide information about the operation of the drive. An operator interface keypad and display unit is often provided on the front of the VFD controller.
When a VFD starts a motor, it initially applies low frequency and voltage to the motor. The starting frequency is typically 2 HZ or less. Starting at such a low frequency avoids the high inrush current that occurs when the motor is started by simply applying a utility voltage by turning on a switch. When a VFD starts, the applied frequency and voltage are increased at a controlled rate or ramped up to accelerate the load without drawing excessive current. This starting method typically allows a motor to develop 150% of its rated torque while drawing only 150% of its rated current. When a motor is simply switched on at a full voltage, it initially draws at least 500% of its rated current at a very low PF while producing less than 150% of its rated torque. As the load accelerates, the available torque usually drops a little and then rises to a peak while the current remains very high until the motor approaches full speed. A VFD can be adjusted to produce a steady 150% starting torque from standstill right up to full speed while drawing only 150% current. For a pulse width modulated VFD, the input current is not directly related to the output/motor current but only to the total power used by the system. In this case of starting from low speed (say initially 2%), even though the motor current may be 150%, the current draw is ˜150%*2%=3%. The current draw then increases as speed increases directly with the total mechanical power+(motor+VFD) losses. At a running load torque of 50% the maximum current draw is ˜50%.
Variable frequency drives are available with voltage and current rating to match the majority of three phase motors that are manufactured for operation from utility power. VFD controllers designed to operate at 110 volts to 690 volts are often classified as low voltage units. Medium voltage VFD controllers are designed to operate at 3300/6000/6600 volts (50 Hz) or up to 13.8 kV. In some applications, a step up transformer is placed between a low voltage and a medium voltage load.
In the transmission and/or distribution of electric power, it is normal practice to interconnect two or more power transformers in order to conveniently and efficiently deliver electric power. Power transformers used in interconnect systems are normally of the three-phase type. An autotransformer is often used to step-up or step-down voltage. The autotransformer consists of one or more windings wound on a core. A typical single phase autotransformer includes first and second end terminals, or taps, and an intermediate tap at which the voltage output is developed.
In many applications, it is desired to provide motors at remote locations. In these remote locations, the utility power supply may be generally inadequate to supply the needs of a motor. If a very large motor is connected to an ineffective utility power supply, then brownouts or damage to the power grid can occur. This is particularly true during the starting of the motor when the power requirements for starting torque are extreme. In order to start such motors, it is often necessary to draw an extreme amount of current to achieve the necessary breakaway torque.
In pipeline operations, very large compressor motors are utilized. Typically, the pipelines extend through very remote areas. These compressor motors must be powered from the utility power supply or from an attached generator. If a generator is utilized, then it must be oversized in order to meet the breakaway power requirements of the particular motor. In other circumstances, lengthy connections must be made in order to connect such a remote motor to the nearest available power grid. In either of these circumstances, it becomes exceedingly expensive to install such a compressor motor in these remote locations. These compressor motors will have horsepower requirements of up to ten thousand horsepower.
As stated hereinbefore, variable frequency drives have often been used to control the AC motor. The variable frequency drive is designed to connect to the motor and therefore operate at the nominal voltage of the motor. For cost and reliability purpose, there are occasions where it is desirable to use a transformer to adapt the variable frequency voltage to the motor (i.e. 600 volts to 4 kV). FIG. 1 illustrates such a system. In FIG. 1, it can be seen that the 690 volt variable frequency drive 10 is connected by line 12 across a transformer 14. The transformer 14 is then connected by line 16 to the 3000 horsepower motor 18. This system of the prior art has two major problems. First, the combination of motor cable capacitance and transformer inductance can cause a doubling of peak voltage at the motor terminals. Secondly, at low frequencies, the motor and transformer resistance define the voltage. For a direct motor connection, the voltage is boosted (increased) but the low frequency operation of the transformer 14 is impossible. The result is a restricted low frequency performance, i.e. low breakaway motor torque.
The increasing use of electric motor-driven reciprocating compressors over gas-driven engines because of enviromental and economic considerations has presented pipeline operators with a new set of motor starting challenges at remote sites having weak electrical supplies. FIG. 2 illustrates the motor torque required relative to the motor speed for a typical unloaded compressor. As can be seen in FIG. 2, after breakaway (“stiction”), the torque requirement is very low. The torque will increase with the speed of the motor. At start, nearly 25% of the motor rated torque is required.
The conventional starting method for a medium voltage (2300 volts and 4000 volts) induction motor is to start them across the power line. This typically results in a 600% inrush current while the motor is accelerating. As can be seen in FIG. 3, the breakaway or locked rotor torque for a Nema B (Starting Type F) motor is around 80%.
At remote sites, where compressors are often located, with long power line feeds, the utility will often not permit the use of high current “across-the-line” starters. The usual approach is to use a reduced voltage soft starter. This reduces an inrush current to about 250 to 300%. However, there is a consequential significant decrease in starting torque to around 15%. This applies to both solid-state soft starters and autotransformer starters. FIG. 4 illustrates these requirements. The net result of this reduced starting torque is that the motor will not be able to break the “stiction” and would not be able to accelerate the compressor.
It is an object of the present invention to provide a soft start system that reduces starting inrush current to minimal levels.
It is another object of the present invention to provide a soft start system which provides sufficient breakaway torque to the motor.
It is another object of the present invention to provide a soft start system that is adaptable to the use of multiple motors from a single starter.
It is still another object of the present invention to provide a soft start system that maximizes the number of starts per hour that are available.
It is still another object of the present invention to provide a bumpless transfer to line.
It is a further object of the present invention to provide a soft start system that is significantly less costly than a fully rated variable frequency drive.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.